At present, in the field of wireless network, the WLAN has been rapidly developed. Demands for WLAN coverage are increasing. The Institute of Electrical and Electronics Engineers (IEEE) industry specification IEEE802.11 working team has defined a series of WLAN technical standards, such as 802.11a, 802.11b and 802.11g. Then, other task groups emerge, which are committed to developing the specifications for improving the existing 802.11 technology. For is example, the 802.11n task group puts forward the requirements of High Throughput (HT), for supporting the data rate which reaches up to 600 Mbps. As the evolution of 802.11n, the 802.11ac task group further puts forward the concept of Very High Throughput (VHT), and increases the data rate up to 1 Gbps. The new protocol needs to be backward compatible with the existing protocol.
In 802.11, a Basic Service Set (BSS) consists of an Access Point (AP) and a plurality of non-access Stations (STAs) associated with the AP. The 802.11 defines two operation modes: a Distributed Coordination Function (DCF) and a Point Coordination Function (PCF), and defines the improvements aiming at the two operation modes: an Enhanced Distributed Coordination Access (EDCA) function and a Hybrid Coordination Function Controlled Channel Access (HCCA) function.
The 802.11 protocol defines the concept of frequency planning class. Each frequency planning class has a sequence number which is corresponding to the operation rules of a group of wireless devices, such as the channel bandwidth, channel behavior restrictions, channel starting frequency points and channel sets. With the evolution of the 802.11 protocol, the traditional 20 MHz channel bandwidth has been expanded to be 40 MHz, 80 MHz, 120 MHz, and even 160 MHz. The large bandwidth is formed by binding a plurality of 20 MHz bandwidths. The 20 MHz channel is named as a basic channel here. When the bandwidth is a large bandwidth, one of the 20 MHz channels is named as a main channel or the first channel, and other 20 MHz channels are named as auxiliary channels or the second channel, the third channel, and the like. Thus, on the 5 GHz frequency spectrum, the large bandwidth is formed by a plurality of 20 MHz channels which are non-overlapping. According to the channelization method in the related art, the 160 MHz channel is composed of two 80 MHz channels which are adjacent or non-adjacent. Each 80 MHz channel is composed of two adjacent 40 MHz channels. Each 40 MHz channel is composed of two adjacent 20 MHz channels. The combination examples of the different bandwidth channels in the large bandwidth system are as shown in FIG. 1. In this case, there are two channelization methods in the frequency band between 5.725 GHz and 5.850 GHz. When the center frequency point starts from 5.745 GHz, and the frequency band is divided on the basis that each channel bandwidth is 20 MHz, there are five 20 MHx channels which are non-overlapping with each other in the frequency band, and the maximum channel bandwidth is 80 MHz. If each of the center frequency points shifts 7.5 MHz towards the direction of the low frequency point, six 20 MHz channels which are non-overlapping with each other can be generated in the frequency band, and the maximum channel bandwidth is 120 MHz. The diagram of the two channelization methods is as shown in FIG. 2. Hereinafter, the channel formed by the former channelization method is named as the first channel set, and the channel formed by the later channelization method is named as the second channel set.
According to the related art, in the frequency band between 5.725 GHz and 5.850 GHz, the 802.11n device is only operated on the first channel set, and the operation bandwidth is 20/40 MHz. The 802.11ac device which does not support the second channel set is only operated on the first channel set, and the operation bandwidth is 20/40/80 MHz. The 802.11ac device which supports the second channel set can be selectively operated on the first channel set or the second channel set. When operated on the second channel set, the operation bandwidth is 20/40/80/120 MHz.
However, the inventors find that in the related art, if one AP of the 802.11ac which supports the second channel set starts one BBS on the second channel set, then only the STA which supports the second channel set can scan the working frequency point of the AP and can be accessed. The STA of the 802.11ac and the STA of the 802.11n which do not support the second channel set cannot be accessed. Thus, it causes the problem that the 802.11ac system cannot be backward compatible.